shmget
SHMGET(2) Linux Programmer’s Manual SHMGET(2)
NAME
shmget - allocates a shared memory segment
SYNOPSIS
#include <sys/ipc.h>
#include <sys/shm.h>
int shmget(key_t key, size_t size, int shmflg);
DESCRIPTION
shmget() returns the identifier of the shared memory segment associ-
ated with the value of the argument key. A new shared memory segment,
with size equal to the value of size rounded up to a multiple of
PAGE_SIZE, is created if key has the value IPC_PRIVATE or key isn’t
IPC_PRIVATE, no shared memory segment corresponding to key exists, and
IPC_CREAT is asserted in shmflg (i.e. shmflg&IPC_CREAT isn’t zero).
The value shmflg is composed of:
SHM_HUGETLB used for allocating HUGETLB pages for shared memory.
IPC_CREAT to create a new segment. If this flag is not
used, then shmget() will find the segment associated with
key and check to see if the user has permission to access
the segment.
IPC_EXCL used with IPC_CREAT to ensure failure if the segment
already exists.
mode_flags (lowest 9 bits)
specifying the permissions granted to the owner, group,
and world. Presently, the execute permissions are not
used by the system.
If a new segment is created, the access permissions from shmflg are
copied into the shm_perm member of the shmid_ds structure that defines
the segment. The shmid_ds structure has the following form:
struct shmid_ds {
struct ipc_perm shm_perm; /* operation perms */
size_t shm_segsz; /* size of segment (bytes) */
time_t shm_atime; /* last attach time */
time_t shm_dtime; /* last detach time */
time_t shm_ctime; /* last change time */
unsigned short shm_cpid; /* pid of creator */
unsigned short shm_lpid; /* pid of last operator */
short shm_nattch; /* no. of current attaches */
};
struct ipc_perm {
key_t key;
ushort uid; /* owner euid and egid */
ushort gid;
ushort cuid; /* creator euid and egid */
ushort cgid;
ushort mode; /* lower 9 bits of shmflg */
ushort seq; /* sequence number */
};
When creating a new shared memory segment, the system call initializes
the shmid_ds data structure shmid_ds as follows:
shm_perm.cuid and shm_perm.uid are set to the effective user-ID
of the calling process.
shm_perm.cgid and shm_perm.gid are set to the effective
group-ID of the calling process.
The lowest order 9 bits of shm_perm.mode are set to the lowest
order 9 bit of shmflg.
shm_segsz is set to the value of size.
shm_lpid, shm_nattch, shm_atime and shm_dtime are set to 0.
shm_ctime is set to the current time.
If the shared memory segment already exists, the access permissions
are verified, and a check is made to see if it is marked for destruc-
tion.
SYSTEM CALLS
fork() After a fork() the child inherits the attached shared memory
segments.
exec() After an exec() all attached shared memory segments are
detached (not destroyed).
exit() Upon exit() all attached shared memory segments are detached
(not destroyed).
RETURN VALUE
A valid segment identifier, shmid, is returned on success, -1 on
error.
ERRORS
On failure, errno is set to one of the following:
EINVAL if a new segment was to be created and size < SHMMIN or
size > SHMMAX, or no new segment was to be created, a seg-
ment with given key existed, but size is greater than the
size of that segment.
EEXIST if IPC_CREAT | IPC_EXCL was specified and the segment
exists.
ENOSPC if all possible shared memory id’s have been taken
(SHMMNI) or if allocating a segment of the requested size
would cause the system to exceed the system-wide limit on
shared memory (SHMALL).
ENOENT if no segment exists for the given key, and IPC_CREAT was
not specified.
EACCES if the user does not have permission to access the shared
memory segment.
ENOMEM if no memory could be allocated for segment overhead.
NOTES
IPC_PRIVATE isn’t a flag field but a key_t type. If this special
value is used for key, the system call ignores everything but the low-
est order 9 bits of shmflg and creates a new shared memory segment (on
success).
The followings are limits on shared memory segment resources affecting
a shmget call:
SHMALL System wide maximum of shared memory pages: policy depen-
dent.
SHMMAX Maximum size in bytes for a shared memory segment: imple-
mentation dependent (currently 4M).
SHMMIN Minimum size in bytes for a shared memory segment: imple-
mentation dependent (currently 1 byte, though PAGE_SIZE is
the effective minimum size).
SHMMNI System wide maximum number of shared memory segments:
implementation dependent (currently 4096, was 128 before
Linux 2.3.99).
The implementation has no specific limits for the per process maximum
number of shared memory segments (SHMSEG).
BUGS
The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would
more clearly show its function.
CONFORMING TO
SVr4, SVID. SVr4 documents an additional error condition EEXIST.
Until version 2.3.30 Linux would return EIDRM for a shmget on a shared
memory segment scheduled for deletion.
SEE ALSO
ftok(3), ipc(5), shmctl(2), shmat(2), shmdt(2)
Linux 0.99.11 1993-11-28 SHMGET(2)
